96 Herbert Eugene Walter 



The foregoing examples illustrate only a few of the many modi- 

 fications of light responses due to the interference of some other 

 stimulus. 



Summary. In judging the effect of any stimulus upon an ani- 

 mal it is necessary to have constantly in mind the accelerating or 

 inhibiting effects of other stimuli which may be influencing the 

 organism at the same time. In the case of planarians some of the 

 responses known to be intimately connected with phototaxis are 

 geotaxis, thigmotaxis, goniotaxis and chemotaxis. 



Planaria gonocephala shows itself to a certain extent negatively 

 geotactic after several hours of dark and positively geotactic after 

 a similar interval of light. 



When given horizontal and vertical surfaces of equal extent, 

 worms travel more on the vertical surfaces. 



Their accuracy in orienting themselves to light while subjected 

 to geotactic stimulus on a vertical surface is less than when they 

 are traveling on a horizontal surface, where the directive geotactic 

 stimulus is eliminated. 



Thigmotactic stimulus may result either from an environmental 

 change in the substratum, or a change in contact caused by the 

 worm itself whereby its relation with the substratum is varied. 



There is a close interdependence of the various stimuli which 

 may be acting on an animal at the same time. 



Behavior may be the direct consequence of light or the indirect 

 result of light combined with the direct effect of a thigmotactic 

 stimulus indirectly brought about by some change in the intensity 

 of the light. 



The greater the degree of contact with the substratum the more 

 responsive a planarian becomes to the mechanical stimulus of jar- 

 ring, but the less to the stimulus of light, as shown by comparing 

 the behavior of worms on the surface film with their behavior on 

 the aquarium floor. 



Goniotaxis has an inhibitive effect on phototaxis; this effect 

 becomes more apparent as the worm reaches a condition of fatigue, 

 phototaxis meanwhile becoming less apparent. 



Dendrocoelum lacteum exhibits a remarkably delicate response 

 (Chemotaxis ?) in frequently coming to rest in the neighborhood 

 of its own kind. 



